The invention relates to a level indicator for liquid contained in a reservoir, and more particularly for the oil level in the sump of an internal combustion engine.
More particularly, the invention relates to a level indicator for liquid contained in a reservoir of the type comprising a probe for immersion in the liquid, a gas pressure with a heating resistance and a liquid column manometer. The gas pressure source comprising a housing containing a gas-filled chamber which communicates with an external gas reserve by way of a filling orifice, with the probe by way of an inlet orifice, and with the manometer by way of a measuring orifice. Communication between the chamber and the probe is controlled by a first valve situated in the inlet orifice.
In level indicators of this type, the probe is usually a tube of which one end is immersed in the liquid whose level is to be monitored. The general principle underlying this type of indicator is that the liquid in the probe is expelled until a pressure equilibrium is reached. This pressure equilibrium corresponds to the liquid level above the end of the probe, and it is measured by means of the liquid column manometer.
When a probe of this type is used to measure the oil level in the sump of an internal combustion engine, running of the engine heats the ambient air and in particular the air trapped in the immersed tube. This heating expands the air enclosed in the tube, causing a portion of the air to escape from the tube in the form of bubbles inside the crankcase. If, however, the engine is switched off for a long time and cools, for example overnight or when the vehicle is unused over a long period, the air trapped in the immersed tube contacts and occupies a smaller volume than it did when the engine was running. The oil level in the tube thereafter rises substantially and irreversibly, since the valve connecting the tube to the interior of the gas pressure source is closed. This means that when the engine is started again the first oil level reading after starting is incorrect.